Air delivery system for regulating thermal growth

ABSTRACT

A timing valve is responsive to certain hydraulic rotor speed input signals so as to schedule increased temperatures of air to a turbine shroud support in accordance with the thermal time constants of the rotor. Clearance between the rotor and the shroud is thereby minimized during both transient and steady-state operating conditions. Temperatures are incrementally increased by selectively combining air from the compressor fifth and ninth stage bleed manifolds. Under certain operating conditions, the timing valve is pre-empted and air is provided at a predetermined temperature level as a function of rotor speed only.

BACKGROUND OF THE INVENTION

This invention relates generally to gas turbine engines and, moreparticularly, to an apparatus for minimizing rotor/shroud clearanceduring both steady-state and transient operation.

As turbine engines continue to become more reliable and efficient bychanges in methods, designs and materials, losses which occur fromexcessive clearances between relatively rotating parts become moreimportant in the many design considerations. In many turbine engineapplications, there is a requirement to operate at variable steady-statespeeds and to transit between these speeds as desired in the regularcourse of operation. For example, in a jet engine of the type used topower aircraft, it is necessary that the operator be able to transit toa desired speed whenever he chooses. The resulting temperature and rotorspeed changes bring about attendant relative growth between the rotorand the surrounding shroud and, in order to maintain the desiredefficiency, this relative growth must be accommodated. The primaryconcern is in maintaining a minimum clearance between the stator androtor while preventing any frictional interference therebetween whichwould cause rubbing and resultant increase in radial clearance duringsubsequent operation. When considering the transient operatingrequirements as mentioned hereinabove, the relative mechanical andthermal growth patterns between the rotor and the shroud present a verydifficult problem.

Various schemes have been devised to variably position the stationaryshroud in response to engine operating parameters in order to reducerotor/shroud clearance. One such scheme is that of the thermal actuatedvalve as described in U.S. Pat. No. 3,966,354, which is assigned to theassignee of the present invention. In that apparatus, a valve operatesin response to the temperature of the cooling air and, to the extentthat the cooling air temperature is dependent on the speed of theengine, the transient condition is considered. However, such a systemtends to be relatively slow in responding and relatively inaccurate intrying to match relative growth during transient operation.

Probably the primary reason that a cooling air system operating only ona speed responsive schedule is inadequate is that such a system is notcapable of taking into account the thermal heating and cooling timeconstants of the rotor for all possible sequences of transitionaloperation. That is, present systems are only capable of matching thermaltime constants of the rotor when the sequence of transient conditionoperation is known. This, of course, is not acceptable since theparticular operating mode and sequence of operation is going to dependon the mission at hand.

It is therefore an object of the present invention to provide anefficient turbine engine which is capable of transiting between variousspeeds while maintaining a minimum clearance between its rotor andshroud.

Another object of the present invention is the provision in a turbineengine for responsively modulating the position of a rotor shroud inresponse to multiple steady-state and transient operation conditions.

Yet another object of the present invention is the provision in aclearance control system for selectively varying the shroud position inresponse to the thermal time constants of the rotor.

Still another object of the present invention is the provision for arotor/shroud clearance control system which is responsive and effectiveover a wide range of steady-state and transient operating conditions.

Yet another object of the present invention is the provision for arotor/shroud clearance control system which is economical to manufactureand relatively simple in operation.

These objects and other features and advantages become more readilyapparent upon reference to the following description when taken inconjunction with the appended drawings.

Briefly, in accordance with one aspect of the invention a timing valveacts in response to rotor speed signals to schedule temperature changesto the flow of air to the turbine shroud support so as to match thethermal time constants of the rotor. In this way, the clearance betweenthe rotor and shroud can be minimized during both transient andsteady-state operation.

By another aspect of the invention the timing valve is activated uponthe reaching of a predetermined level of rotor speed. It then advancesat a constant rate to incrementally schedule increases in thetemperature of the air.

By another aspect of the invention the air temperatures are varied bythe use of two air sources at different temperatures. They areselectively used independently or mixed so as to obtain air at fourdifferent modes of airflow.

By still another aspect of the invention the timing valve, upon therotors decreasing to a predetermined speed level, begins to retract at aconstant speed toward its original position. The speed of retraction isslower than the speed of advance so as to accommodate rebursts withoutrotor-to-shroud interference. During the retraction phase, the airdelivery is determined by rotor speed and is independent of timing valveposition.

By another aspect of the invention the timing valve continues to advancefor a predetermined time after reaching the highest temperature mode ofairflow so that during the retraction period, the resulting additionaltime allows the rotor to cool sufficiently to permit rotor burstswithout attendant rub.

By yet another object of this invention the scheduling function of thetiming valve is pre-empted by the rotor's operation at predeterminedspeeds. The temperature of the air delivered is then determined solelyby the rotor speed speed and irrespective of the position of the timervalve.

In the drawings as hereinafter described, a preferred embodiment isdepicted; however, various other modifications and alternateconstructions can be made thereto without departing from the true spiritand scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial cross-sectional view of the turbine shroud supportportion in accordance with the preferred embodiment of the invention;

FIG. 2 is a schematic illustration of the turbine shroud cooling systemin accordance with the preferred embodiment of the invention;

FIG. 3 is a cross-sectional view, partially shown in schematic, of thespeed sensing portion of the preferred embodiment of the invention;

FIG. 4 is a cross-sectional view, partially shown in schematic, of thetimer and air valve portion of the invention;

FIG. 5 is a table showing the steady-state modes of operation of thepresent invention;

FIG. 6 is a graphic illustration showing the relationship of the variousparameters during steady-state operation;

FIG. 7 is a graphic illustration of various sequence valve positions andair valve positions as a function of time; and

FIGS. 8a through 8m are schematic illustrations of the sequence of valveand air valve positions as they are sequenced through a typical cycle ofoperation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 there is shown a portion of a typical gasturbine engine which includes a row of circumferentially spaced highpressure turbine blades 11 which are closely circumscribed by aplurality of circumferentially spaced shroud segments 12. As inconventional operation of a single-stage high pressure turbine, the hotexhaust gases from the combustor (not shown) pass through a row of highpressure nozzles 13, through a row of turbine blades 11 to impart rotarymotion thereto, and downstream to a row of low pressure nozzles 14.Cooling air is provided to the high pressure nozzles 13 and the lowpressure nozzles 14 by way of cooling plenums 16 and 17, respectively,in a manner well known in the art.

The shroud segments 12 are supported by a shroud support structure 18having inner flanges 19 and 21 which are interconnected with the shroudsegments by way of an annular clamp bracket 22 and support bracket 23,respectively. In order to cool the shroud segments 12, it is common topass cooling air from a cavity 24 through the support bracket 23 to thecavity 26 where it passes through perforations in a baffle 27 to impingeon and cool the shroud segments 12.

The shroud support ring 18, which is supported at its forward end byattachment to the combustor casing 28 and, at its rear end by attachmentto the nozzle support element 29 and the low pressure turbine casing 31,includes a middle flange 32 and an aft flange 35. These flanges are ofsubstantial thickness and radial height such that they represent asubstantial portion of the mass of the entire shroud support structure18. It will be understood that by selectively controlling thetemperature, and thus the thermal growth, of these flanges, the radialposition of the shroud segment structure 18 and thus that of the shroudsegments 12 can be modulated to follow the mechanical and thermal growthof the rotor blade 11 in order to minimize the clearance between theblades 11 and the shroud segments 12 during both steady-state andtransient operating conditions.

Surrounding the shroud support structure is a manifold 33 which isconnected at its forward end to a combustor casing 28 by a plurality offasteners 34 and, at its rearward end to the turbine casing 31 by aplurality of fasteners 36. The manifold 33 defines the outer side of ahigh pressure cooling air plenum 37 and a lower pressure cooling airplenum 38, with the two plenums 37 and 38 being separated by a wall 39which may include provision for the flow of some air between the plenumsas shown by the arrows. Thus, the lower pressure cooling air plenum 38may be supplied with cooling air by way of the high pressure cooling airplenum 37, by air crossing the top of flange 19 after impingement,and/or by way of a separate supply conduit 41 as shown. Cooling of thelow pressure nozzles is accomplished by a manner well known in the art.

Leading into the high pressure cooling air plenum 37 is a bleed airconduit 42 which receives bleed air from the compressor, at varyingtemperatures, in a manner to be more fully described hereinafter.Defining the radially inner boundary of the cooling air plenum 37 andthe radially outer boundary of an inner plenum 43, is an impingementring 44 having a plurality of circumferentially spaced holes 46 disposedtherein for impinging the relatively higher pressure air from the plenum37 against the surfaces of the middle flange 32 and the aft flange 35 tocontrol the temperature thereof. The impinged air then passes from therelatively intermediate pressure plenum 43 in a conventional manner forcooling other elements of the engine.

Scheduling the flow of cooling air to the high pressure cooling airplenum 37 by way of the bleed air conduit 42 is accomplished by way ofthe system shown generally in FIG. 2. Here a main fuel control 47, whichreceives an input indicative of the rotor speed, provides a plurality ofhydraulic pressure outputs that are used to operate a timer 48 and apair of air valve actuators 49 and 51 with air valves 55 and 60,respectively, for the scheduling of air to a manifold 52 and hence tothe shroud support by means of duct 42.

Within the main fuel control 47 there is a pair of pressure balancedhydraulic signal valves 53 and 54 (see FIG. 3) with plungers 56 and 57,respectively, whose positions are controlled by a cam 58 which engagesthe respective valve stems 59 and 61. The cam 58 is positioned inresponse to a core speed tachometer which is normally used forscheduling acceleration fuel flow and compressor stator position. Thistwo-valve system incorporates a specific speed hysteresis band at eachswitch point to prevent the system from alternating back and forthbetween modes when operating near a switch point speed. The cam profiledrives the inner valve stem of each of the valves such that once theswitch point displacement is achieved, the trapped plunger moves by thedifferential pressure to the opposite extreme of its travel within thestops of the valve stem. Therefore the speed must change by the amountcorresponding to the plunger travel before the signal returns to itsoriginal valve clearances. Since the travel of the plunger is limited tothe range corresponding to the speed hysteresis band, the resultingeffect upon the shroud clearances is minimal.

Each of the signal valves 53 and 54 has three hydraulic pressure inputswhich are readily obtainable from the existing state-of-the-art fuelcontrol system. The inputs are P_(B) (boost pressure), P_(CR) (P_(B)+100 psi) and P_(C) (P_(B) +200 psi). The respective plungers 56 and 57are then positioned by the cam 58 to obtain combinations of thesepressures in the respective signal valves to produce output turbineclearance signals TC₂ and TC₁ which will be equal to either P_(B) orP_(C). These two hydraulic signals TC₁ and TC₂, along with the hydraulicsignal P_(B), are then sent to the timer 48 which in turn generateshydraulic signals along lines 62 and 63 to operate the air valveactuators 51 and 49, respectively. These valve actuators also receive aP_(CR) pressure input signal for use as a reference pressure. They thenoperate the air valves 60 and 55 in response to the hydraulic signalsfrom the timer 48 to present different combinations of fifth and ninthstage cooling air to the air valve discharge port or manifold 52 for usein controlling the temperature of the shroud support.

Referring now to FIG. 4, the timer apparatus 48 is shown to have atwo-diameter cylinder 64 and a two-diameter piston 66. Disposed in thecylinder larger end 67 between its one wall 68 and the piston larger end69 is a helical spring 71 which tends to bias the piston toward the headend cavity 70 in the left end of the cylinder 64. The piston smaller end72 includes three axially spaced lands 73, 74 and 76 which extendradially outward to be in close diametral clearance with the internalwall of the cylinder smaller end 77. The piston 66 has a passage 78extending axially from one end to the other, an orifice 80 at its oneend to meter the fluid flow to the head end cavity 70, and a port 79which fluidly connects the passage 78 to a cavity 81 between the lands74 and 76. The piston smaller end 72 also includes a passage 82 whichfluidly connects a cavity 83 between the lands 73 and 74 to the cylinderlarger end 67.

Hydraulic connections to the cylinder 64 are made at its smaller end bylines 84, 86 and 87 and to its larger end by line 88. The line 84 comesinto the end of the cylinder smaller portion 77 and carries a hydraulicfluid at a pressure designated TC₂ which emanates from the signal valve53 located in the main fuel control 47. Line 86 is connected at its oneend to the side of the cylinder smaller portion 77 and, at its other endto one end of a maximum pressure selector 89. Line 87 is connected atits one end to the cylinder smaller end 77 and, at its other end, to oneend of a maximum pressure selector 91. Line 88 is connected at its oneend to the wall 68 of the cylinder larger end 67 and at its other end toa mixing valve 92. The mixing valve 92 comprises a cylinder 93 having adouble-ended piston 94 disposed therein and a helical spring 96 biasingthe piston 94 in the downward position as shown. The upper end of thecylinder 93 is fluidly connected by line 97 to the signal valve 53 ofthe main fuel control 47 to present a TC₂ fluid pressure thereto. Thelower end of the mixing valve 92 is connected by hydraulic line 98 tothe other signal valve 54 in the main fuel control 47 to receive the TC₁fluid pressure signal. The hydraulic line 88, which connects to thecylinder larger end 67 comes into the mixing valve 92 at a pointintermediate the two ends, and another hydraulic line 99 carrying fluidat a pressure P_(B) also comes into the cylinder 93 at substantially thesame axial point. Finally, a hydraulic line 101 leads from a point atthe lower end of the cylinder 93 to the other end of the maximumpressure selector 89. Operation of the mixer valve 92 under varyingconditions will be described hereinafter.

Referring now to the maximum pressure selector valves 89 and 91, thereare included the respective balls 102 and 103 whose positions aredetermined by the pressures acting thereon such that they allow only thehighest pressure to which they are exposed to flow to the respective airvalve actuators 51 and 49. For example, in the maximum pressure selectorvalve 89, the ball 102 is exposed to the pressures in lines 86 and 101and moves to allow only the higher pressures to enter the line 62 andinto the one end of the air valve actuator 51. Similarly, the valve 91operates to only allow the higher pressure from the line 87 and the line98 to flow into the line 63 and into the one end of the air valveactuator 49.

As shown in FIG. 4, the air valve 55 is normally closed and its actuator49 is biased by a helical spring 104 as well as a hydraulic fluid at apressure P_(CR). The air valve 60 is a normally open valve and itsactuator 51 is biased by a helical spring 106 and hydraulic fluid at apressure of P_(CR).

Operation of the timer 48 at steady-state idle speed condition will nowbe described. At idle speed, the cam 58 (see FIG. 3) moves the plungers56 and 57 to the positions such that the low pressure P_(B) exists atboth the TC₁ and TC₂ signal points. The low fluid pressure then existswithin the line 84 shown in FIG. 4, cylinder smaller end 77, the passage78 and the head end cavity 70, to act on the piston larger end 69. Theother side of the piston 69 larger end is acted upon by the same lowP_(B) pressure from line 99 through mixing valve 92 by means of line 88.Since the pressures are equal, the piston 66 remains in that position.The same low pressure exists in line 86 and, since the fluid is enteringthe mixing valve 92 by way of line 99 at a pressure P_(B), the pressurein line 101 is also at a low P_(B) pressure. Accordingly, the pressurein line 62 is at P_(B) and the spring 106 and the fluid at pressureP_(CR) hold the actuator 51 retracted and the valve 60 in the openposition to allow for the flow of ninth stage air to the valve dischargeport 52.

At the same time, the fluid at a low pressure P_(B) exists in the lines98 and 87 to the maximum pressure selector 91 and in the line 63 at theleft end of the air valve actuator 49. The other side of the air valve49 actuator has the force of spring 104 and fluid pressure P_(CR) on itand therefore the valve 55 is held in its normally closed position toprevent the flow of fifth stage air into the air valve discharge port52.

The mixing valve 92 has hydraulic fluid at a low pressure P_(B) in bothends thereof and therefore the piston 94 remains in its downwardlybiased position as shown.

Operation of the system in the steady-state idle mode of operation isintended to employ relatively hot ninth-stage air to establish theshroud position promptly and provide the necessary clearance margin foroperation as will be more fully discussed hereinafter. In addition tothe idle mode of operation, the system is designed to operate over theentire core speed range, and for the purpose of this description isillustrated for a standard day in terms of the steady-state operatingmodes, cruise, climb and takeoff as shown in the table of FIG. 5. Itwill be recognized that these modes for steady-state operation employincreasingly hotter air as the engine speed and temperatures increase inorder to match the thermal growth of the rotor. That is, after theinitial start and idle operation, the coolest source of air from thefifth stage is used in the cruise range of 10,000 to 13,400 rpm, thenthe fifth and ninth stages are mixed for the climb range of 13,400 rpmto 14,000 rpm and finally only the ninth stage air is used in thesteady-state takeoff mode of operation, above 14,000 rpm so as to ensureadequate clearance during hot day takeoff operation.

It will be recognized by reference to the table of FIG. 5 that for boththe idle and cruise modes of operation, the timer piston 66 will remainin the retracted position as shown in FIG. 4 or, as will be more clearlyseen hereinafter, if the engine has been operating at a higher speed anddecreases to either of those speed ranges then the timer piston 66 willbe retracting such that it is moving leftward to the position as shownin FIG. 4. In the other two modes of operation, for climb and takeoff,the timer piston 66 is advancing to schedule the increasing airtemperatures as indicated.

There is shown graphically in FIG. 6 the various control signals, thepiston final positions, the air sources applied and the rotor-to-shroudclearances which correspond to the respective steady-state modes ofoperation. During the idle mode, both the TC₁ and TC₂ have the low P_(B)pressure signals so that the piston is in the far left position. Onlythe ninth stage air is turned on such that the rotor-to-shroud clearanceis at a maximum level possible for this speed. As the speed is increasedto the cruise range, the TC₁ control signal is increased to the P_(C)level such that the ninth stage air is turned off and the fifth stageair is turned on. This results in a substantial decrease in therotor-to-shroud clearance, and this clearance continues to decrease asthe speed is increased to the climb range. At that point, the TC₂ signalis increased to the P_(C) level and the sequence timer piston moves toits final position to the far right. The ninth stage air is then turnedon to present a mixture of fifth and ninth stage air and increase therotor-to-shroud clearance to an acceptable level as shown. Again, thisclearance decreases as speed increases to the takeoff level wherein theTC₁ signal is decreased to the P_(B) level and the fifth stage air isremoved to again present an increased rotor-to-shroud clearance so as toallow for further increases of speed without attendant rub.

Considering now the operation of the system under transient conditions,there is shown in FIG. 7 a graphic illustration of the timer valvepiston positions and air valve positions for an engine which operatesfrom the idle condition to the takeoff position and back down to thecruise/idle condition. In following through these sequences, the valvepositions will be examined in relation to the controlling parameters andthe time by reference to FIGS. 8a through 8m.

As illustrated in FIG. 4, when the engine is in the idle position, theninth stage air is being delivered to the shroud support to provide anadequate clearance between the rotor and the shroud. When the engine isaccelerated to the takeoff range, the system will begin to function asshown in FIG. 8a and, as time progresses, it will sequence through thefunctions as illustrated by FIGS. 8b, 8c and 8d.

Referring now to FIG. 8a, the hydraulic signal TC₁ is at a low P_(B)level and the signal TC₂ is at a high P_(C) level. Accordingly, the highpressure fluid in the line 84 passes through the passage 78, the orifice80, and into the head end cavity 70 at the left end of the piston 66 tocause it to start moving to the right. At the same time, the highpressure fluid in the line 86 moves the ball 102 of the pressureselector 89 down to allow the high pressure fluid to flow into the line62 to act against the spring 106 and the P_(CR) pressure of the airvalve actuator 51 to extend the piston and to close off the ninth stageair. The remaining portion of the circuit remains at a low pressureP_(B) condition. This no-flow condition is represented by the first 30seconds of operation wherein the timer valve piston moves 20% of itstravel to the right as indicated in FIG. 7.

After 30 seconds, the piston 66 has moved to the right to the positionas shown in FIG. 8b. At this point the high pressure fluid enters theport 79 and the cavity 81 to then flow into the line 87 where the ball103 in the maximum pressure selector 91 is moved down as shown. The highpressure fluid enters the line 63 and overcomes the spring force andP_(CR) pressure to extend the actuator 49 and to open the normallyclosed valve 55 which allows the fifth stage air to enter the valvedischarge port 52. This condition exists for fifteen seconds as thepiston 66 moves to the 30% position as shown in FIG. 7.

After forty-five seconds, the land 73 of the piston 66 passes to theright of the port going to the line 86 and thereby cuts off the supplyof high pressure fluid to that line (see FIG. 8c). The line 86 is thenexposed to a low pressure P_(B) fluid which enters by way of the line88, the cylinder large end 67, the passage 82, and the cavity 83. Thepressure in the line 62 then drops to the low P_(B) level and the P_(CR)pressure retracts the actuator to move the air valve 60 to the normallyopen position. The ninth stage air then comes into the discharge port 52to present a fifth and ninth stage mixture which remains for the nextforty seconds as the piston 66 advances to the fifty-seven percentposition as shown in FIG. 7.

After a total of 85 seconds (see FIG. 8), the land 76 passes to theright of the port entering the line 87 and a low pressure P_(B) fluidexists in the line 88, the cylinder large end 67, to present a lowpressure condition to the maximum pressure selector 91. Since there is alow pressure presented at both sides of the maximum pressure selector91, a low pressure fluid exists in the line 63 and the P_(CR) pressurereturns the actuator 49 to the left to place the air valve 55 to itsnormally closed position as shown to shut off the supply of fifth stageair. As indicated in FIG. 7, this condition exists between theeighty-five second and one hundred fifty second time period as thepiston 66 moves to the full right position. This continued advancementbeyond the point where the air valve 55 is closed, is referred to asovertravel and is included in the design to extend the retraction timeof the timer piston so as to allow the rotor to cool sufficiently topermit speed rebursts without attendant rub. This will be more clearlydescribed hereinafter.

As long as the rotor speed remains above the 14,000 rpm level, thepiston will remain in the far right position and the ninth stage airwill continue to flow to the shroud support. When the speed is now cutback from the takeoff speed of 14,000 rpm to a reduced speed whichdemands another steady-state mode as, for example, a climb range of13,400 to 14,000 rpm or the cruise range from 10,000 to 13,400 rpm, thenthe system changes to provide different cooling modes. For example, ifthe speed is dropped to within the 13,400 to 14,000 rpm range, a mixedfifth and ninth stage flow will occur immediately. If the speed isdropped down into the cruise range the system will immediately adjust toprovide only fifth stage air to the manifold. At this point, when thespeed drops below the 13,400 rpm level the timer begins to retract andfollow the downward slope as indicated in FIG. 7. Note that the timerpiston takes only 150 seconds to advance completely to the right andtakes 650 seconds to retract completely to the left side. Since therotor takes more time to cool down at low speeds than it takes to heatup at high speeds, this slower retraction is necessary in order toprevent rotor-to-shroud rubs during a reburst to high thrust. As will beseen, the overtravel of the piston for a period of sixty-five secondswill result in an additional retraction time of approximately 280seconds. This time allows the high inertia rotor to cool downsufficiently while the shroud cooling system remains in the hotteroperating modes such that a reburst to climb, for example, will notresult in a rub.

When the speed is reduced to a level below 13,400 rpm, the piston willretract toward the left and the fifth stage air valve will remain openas shown in FIGS. 8e through 8h. Referring to FIG. 8e, when the speeddrops below the 13,400 rpm level, the TC₁ and TC₂ signals switch suchthat the TC₁ is a high P_(C) pressure and TC₂ is a low P_(B) pressure.The higher level TC₁ pressure signal exists in the line 98, in themaximum pressure selector 91 and in the line 63 allowing P_(C) to openthe normally closed air valve 55 to allow the fifth stage air to flowinto the air valve discharge port 52. At the same time, the highpressure signal at TC₁ enters the mixing valve 92 to move the piston 94to the upward position and establish the high pressure fluid in the line101, the maximum pressure selector 89, and the line 62 to close thenormally open air valve 60 to prevent the ninth stage air from enteringthe air valve discharge port 52. The low pressure TC₂ signal P_(B) whichpasses through the passage 78 and orifice 80 to the head end cavity 70is insufficient to overcome the force of the helical spring 71 and thusthe piston 66 begins to retract toward the left. At the end of a280-second period, the piston 66 has come to the position as shown inFIG. 8f wherein the land 76 has passed to the left side of the portentering the line 87 such that the low pressure from line 84 exists inthe port 79, the cavity 81 and the line 87. The ball 103 of the maximumpressure selector 91 is then moved to the up position as shown and stillthe high pressure fluid passes from the line 98 to the line 63 andfinally to the air valve actuator 49 to hold the air valve 55 in theopen position.

After 450 seconds, the piston 66 has moved to the position as shown inFIG. 8g wherein the land 73 has passed to the left side of the portentering the line 86. There remains a low pressure on top of the ball102 and a high pressure below the ball and, in the line 62 and in theair valve 51 to hold the air valve 60 in the closed position.

After 650 seconds of cruise operation, the piston 66 has traveled to thefull left position as shown in FIG. 8h such that the land 74 has passedto the left side of the port entering the line 87 so as to expose thatline to the low pressure P_(B) fluid which travels along line 88 to thepassage 82 and to the cavity 83. Again, the high pressure fluid from theline 98 passes through the maximum pressure selector 91 and the line 63to hold the air valve 55 in the open position. The system will remain inthis condition as long as this cruise speed is maintained.

It should be noted that during the period in which the piston 66 isretracting, as represented by the downward sloping line in FIG. 7, ifthe speed is then increased to above the 13,400 rpm cruise modethreshhold, the piston 66 will then begin to advance back to the righton an advancement schedule as described hereinabove. However, it willpick up where the retraction stopped rather than starting at the farleft position. For example, if after 200 seconds of retraction, thespeed is again increased to the takeoff level of 14,000 rpm, the piston66 will have retracted down to the 70 percent travel position asrepresented by A on the graph of FIG. 7. The piston will then begin toadvance along the advancement schedule from the position B as shown andwill at that time change from fifth stage cooling air only to ninthstage cooling air only.

If the engine is operated at cruise for 200 seconds so that the piston66 is at the position A in FIG. 7, and the engine is then accelerated toa climb range, between 13,400 and 14,000 rpm, then the system willadjust to the condition as shown in FIG. 8i. Here both the TC₁ and theTC₂ signals are at the high P_(C) pressure and the higher pressure fluidwill thus enter the passage 78, the orifice 80 and the head end cavity70 to reverse the direction of the piston 66 and start it back again onits advancement schedule from the point B of FIG. 7. The high pressureTC₁ signal will pass along line 98 through the maximum pressure selector91 through the line 63 and to the air valve actuator 49 to hold thevalve 55 in the open position to permit the flow of fifth stage air. Nowat the mixer valve 92 there is a high pressure at both ends andtherefore the piston 94 will remain in the spring-biased lower positionas shown and the lower pressure signal P_(B) will be present in thelines 101 and 86 and the maximum pressure selector 89 such that thepressure fluid entering the line 62 and the air valve actuator 51 willbe at a low pressure and permit the air valve 60 to open and allow theflow of ninth stage air to the air valve discharge port 52. Thus, thismixed flow mode will continue as long as operation of the engine iswithin the climb range of speeds between 13,400 and 14,000 rpm.

Now if the system has been operating in the cruise mode for the full 650seconds and the piston 66 has thus traveled to the far left position,the fifth stage air will continue to flow until another steady-statemode is called for. If the speed is now advanced to the climb range,between 13,400 and 14,000 rpm, the timer will again begin to advancefrom the zero position of FIG. 7 and will sequence through the variousconditions as shown by FIGS. 8j through 8m. In FIG. 8j, both the TC₁ andTC₂ have the high pressure P_(C) signals. The high pressure TC₂ signalexists in line 84, line 86 and line 62 to hold the air valve 60 in theclosed position. The high pressure TC₁ signal passes along line 98 andline 63 to the air valve actuator 49 to hold the valve 55 in the openposition to allow fifth stage air to enter the air valve discharge port52.

After thirty seconds, the piston 66 has traveled to the position asshown in FIG. 8k wherein the land 74 has passed to the right side of theentrance to the line 87. The high pressure fluid from line 84 thusenters the port 79, the cavity 81 and the line 87, providing highpressure fluid to both sides of the ball 103. Still there is a highpressure in the line 63 and to the air valve actuator 49 to hold thevalve 55 in the open position as before.

After forty-five seconds, the piston 66 has advanced to the position asshown in FIG. 8l wherein the land 73 has passed to the right side of theport entering the line 86. Now the fluid at a low pressure P_(B) fromthe line 88, the passage 82, the cavity 83 and the line 86 provides alow pressure condition in line 62 such that the air valve 60 moves tothe normally open position to also introduce ninth stage air to theavailable discharge port 52.

After eighty-five seconds of operation, the piston 66 has moved to theposition as shown in FIG. 8m wherein the land 76 has passed to the rightside of the port entering the line 87. This presents a low pressurefluid to the maximum pressure selector 91 to allow the ball 103 to moveto the position as shown. However, the high pressure fluid from the line98 still passes to the line 63 and the air valve actuator 49 to hold thevalve 55 in the open position. The piston 66 will commence theovertravel mode with the fifth and ninth stage air mixture presisting aslong as the climb mode of operation is continued. If the speed is thendecreased to the cruise or idle mode, the piston will again go into theretraction schedule on a slope as indicated in FIG. 7.

It will be understood that many combinations of the particular coolingsystem described hereinabove may be selected by one skilled in the artwithout departing from the true spirit and scope of the invention. Forexample, although the invention has been described in terms of operationat particular core speeds and ranges, it may just as well be exercisedby the use of other schedules, speeds and missions to suit anyparticular operating requirement. Further, the schedule may beperiodically modified slightly as necessary to account for performancechanges brought about by age. Other possibilities would include the useof alternate air sources, speed sensing apparatus and/or support coolingarrangements.

Having thus described the invention, what is claimed as novel anddesired to be secured by Letters Patent of the United States is:
 1. Incombination with apparatus of the type having a compressor, a rotor, ashroud, a shroud support surrounding the rotor, an air delivery systemfor providing air at varying temperatures to the shroud supportcomprising:(a) first and second air sources, the temperature of saidsecond air source being higher than that of said first air source; (b)first and second air valves for controlling the flow of air from saidfirst and second air sources, respectively, to a manifold; (c) means fordelivering air from the manifold to the support; and (d) valve meansresponsive to the speed of said rotor and to the time after reachingpredetermined rotor speeds for operating said first and second airvalves, said valve means including a timer valve which is activated whenthe speed of the rotor reaches a predetermined level with said timervalve including a piston which advances at a first substantiallyconstant rate of speed upon receiving a first predetermined rotor speedsignal and which retracts at a second substantially constant rate ofspeed upon receiving a second predetermined rotor speed signal, whereinsaid valve means operates said first and second air valves in responseto translation of said piston.
 2. An air delivery system as set forth inclaim 1 wherein said first and second air sources comprise compressorbleed sources.
 3. An air delivery system as set forth in claim 1 whereinsaid first air source comprises an intermediate compressor stage.
 4. Anair delivery system as set forth in claim 1 wherein said second airsource comprises a later compressor stage.
 5. An air delivery system asset forth in claim 1 wherein said first air valve comprises a normallyclosed valve.
 6. An air delivery system as set forth in claim 1 whereinsaid second air valve comprises a normally open valve.
 7. An airdelivery system as set forth in claim 1 wherein said valve means isresponsive to two hydraulic signals indicative of predetermined speedranges and wherein the difference in pressure of said two hydraulicsignals is substantially constant.
 8. An air delivery system as setforth in claim 1 wherein said piston continues to advance for apredetermined time after receiving a predetermined rotor speed signal.9. An air delivery system as set forth in claim 1 wherein said piston isspring-biased.
 10. An air delivery system as set forth in claim 1wherein said piston is caused to move at a constant speed by fluidpressure acting on one end thereof.
 11. An air delivery system as setforth in claim 1 wherein said piston has a fluid passage extendinglongitudinally therethrough.
 12. An air delivery system as set forth inclaim 1 wherein said valve means provides one of two pressure inputs toeach of said air valves.
 13. In combination with apparatus of the typehaving a compressor, a variable speed rotor, a shroud, a shroud supportsurrounding the rotor, an improved air delivery system for supplying airflow to the shroud support in response to the rotor speed wherein theimprovement comprises:(a) air valve means for selectively modulating thetemperature of the air; (b) means for timing the operation of the rotorafter its acceleration to a predetermined level of operation, said meansincluding a piston responsive to rotor speed which advances at a firstpredetermined speed when the rotor accelerates to a predetermined levelof operation and retracts at a second predetermined speed when the rotordecelerates to a predetermined level of operation; and (c) means formodulating said air valve means in response to translation of saidpiston of said timing means.
 14. An improved air delivery system as setforth in claim 13 wherein said air valve means includes first and secondair valves of the on/off type.
 15. An improved air delivery system asset forth in claim 13 wherein said air valve means includes a normallyopen valve and a normally closed valve.
 16. An improved air deliverysystem as set forth in claim 13 wherein said air valve means controlsthe flow of bleed air from the compressor.
 17. An improved air deliverysystem as set forth in claim 14 wherein said air valve means connectstwo air sources of different temperatures.
 18. An improved air deliverysystem as set forth in claim 15 wherein said two air sources compriseintermediate and latter stage compressor bleed air.
 19. An improved airdelivery system as set forth in claim 13 wherein said timing means isactivated when the speed of the rotor reaches a predetermined level. 20.An improved air delivery system as set forth in claim 13 wherein saidpiston continues to advance for a predetermined time after receiving apredetermined rotor speed signal and wherein said modulating meansoperates for only a portion of said predetermine time.
 21. An improvedair delivery system as set forth in claim 13 wherein said piston isadvanced by a fluid pressure acting on one end thereof.
 22. An improvedair delivery system as set forth in claim 13 wherein said air valvemeans is responsive to pressure input thereto and said modulating meansprovides a reference pressure input to said air valve means.
 23. Animproved air delivery system as set forth in claim 22 wherein saidmodulating means provides both a high and a low pressure input to saidvalve means.